This invention relates to a corrector system for inclusion in an electron optic instrument, particularly a scanning transmission electron microscope, for minimizing odd-order aberrations and thus obtaining greatly improved resolving power.
The history of electron microscopy has been one of searching for methods for achieving higher and higher resolving powers. The upper limit would be the wavelength of the electrons themselves, and consequently probably not attainable. The next nearest goal would be the capability of directly imaging the atoms in a solid. Since the distances involved are of the order of 1-2 Angstrom units and it is necessary to image a projection of the atoms on to a plane, a resolution of 1 Angstrom unit or less would be highly desirable. This goal is beyond the reach of existing microscopes.
The primary reason for the inability to achieve such a resolution is the existence of large lens aberrations, particularly the spherical, or aperture, aberration. Using optimum conditions, the best resolution that can now be obtained is about 2-3 Angstrom units, a value which approximates what is needed but is not small enough to allow direct imaging.
The most commonly employed indirect technique used in conventional microscopes takes advantage of various indirect imaging methods. These usually employ an accurately aligned crystal and are based upon the repetitive nature of the arrangement of the atoms. By carefully controlling the amount of defocus and the many other parameters of the microscope an image can be obtained which represents the atomic positions. Such images need careful interpretation, however, since the image details change drastically with defocus, specimen thickness, voltage, and the like. Such "interferograms" have been used to image lattice spacings below the 1 Angstrom unit level, using microscopes whose nominal resolution for amorphous detail would be no more than 2-3 Angstrom units. Other related techniques involve taking a focal series of a multilayer specimen and comparing the images with computer simulations. The practical limit for this technique appears to have been reached with the proposed installation of a one-million volt microscope, which is designed to have a resolution of about 1.8 Angstrom unit and should be capable of lattice resolution below the 1 Angstrom unit level. This is still far away from the ideal goal of imaging atoms in an "amorphous" specimen in an unambiguous manner with a point resolving power below 1 Angstrom unit.
The difficulties involved in the correction of spherical aberration can easily be inferred from the history of events. It has been demonstrated theoretically that all cylindrically symmetric lenses have the same sign of spherical aberration. By implication then, the spherical aberration could not be made zero, and the only hope of eliminating it would be to build a separate correcting device which could not by cylindrically symmetric.
There was later demonstrated the possibility of a correction device, consisting of four quadrupoles and three octupoles for a total of 40 pole pieces. The effect was successfully demonstrated with such a device although not on a scale sufficiently small to be suitable for microscopy. Scaling these large systems down for an electron microscope would be difficult because theoretical investigations indicated a mechanical tolerance of about 0.1 micron for each of the 40 pole pieces. All attempts to construct such a device have failed.
There remains a need for significant improvement in electron optic instrumentation systems to afford a degree of resolution sufficient to permit direct imaging of atoms in solids, whether crystalline or amorphous.